I. Field of the Invention
The present invention relates generally to inclined axis rotary drum batch mixers for mixing and dispensing concrete. More particularly, the invention relates to an improved configuration of mixing fins deployed within rotary drum batch mixers of the class and which improves overall performance, particularly improving the ability of the mixer to control concrete discharge. The invention further relates to reducing drum weight and maintenance and is particularly applicable to mobile rotary mixing drums carried by concrete mixing trucks.
II. Related Art
Concrete mixing trucks are widely used in the construction industry for preparing and transporting concrete mixtures to desired locations for use. A mixing truck typically includes a rotatable mixing drum which has fins or agitators mounted inside for mixing and directing the movement of the concrete mixture therein. Conventionally, the fins have a spiral or helical configuration, including a plurality of flights, which tends to blend and mix the concrete when the mixing drum is rotated in a first direction and urge the concrete toward a discharge chute when the mixing drum is turned in the opposite direction.
In the past, concrete mixing drums of conventional design have been provided with helical mixing and discharge fins in the form of two spiral mixing flights located symmetrically opposite each other, i.e., rotated 180.degree. apart in the drum structure. These two flights are normally identical to each other in most conventional designs especially toward the discharge end. An exemplary composite non-metallic fin compositions and construction is shown in U.S. Pat. No. 5,178,457 to Helmy. The design of a typical conventional concrete mixing drum of the class and with particular reference to the fin structure is illustrated and described in U.S. Pat. Nos. 5,056,924, 5,378,061 and 5,427,449, all of which are assigned to the assignee of the present application and are deemed incorporated herein by reference for any purpose. That class of design is further illustrated in FIGS. 1 and 2.
FIG. 1 is a side elevational view of a mobile system for mixing and dispensing concrete with a portion of the mixing drum wall cut away to expose the fins. The mobile system includes a mixing truck 10 having a cab portion 12 and a rear portion 14 which has a main frame 16. A mixing drum 18 is mounted for rotation on a front support frame 20 and rear support frame 22, both of which are integral with the main frame 16. A rearward portion of the mixing drum 18 is positioned adjacent a discharge mechanism 24 which includes a funnel for charging concrete components into the mixing drum 18, as well as a portion for discharging mixed concrete into a main chute 26, as is well known in the art. Main chute 26 is supported relative to rear support frame 22 by a pivot joint 28 which, in turn, enables main chute 26 to be positioned over a set of forms or other desired location for use of the mixed concrete.
As may be seen in FIGS. 1 and 2, mixing drum 18 includes a front head cone 31 and front cone 30, a belly or cross-over portion 32, a big cone portion 34 and a rear tail cone portion 35 which terminates at the end of truck 10 and which is proximate the discharge mechanism 24, which is supported by rear support frame 22. A conventional dual helical or spiral mixing fin assembly 36 is mounted to an inner surface of an outer wall and extends transversely into the mixing space of mixing drum 18.
The fin assembly 36 includes a first rearwardly curving segment 37, a second forwardly curved fin segment 39 and a transitional fin portion 41 which connects the rearwardly curving segment 37 and the forwardly curving segment 39. As can be seen from the figures, a concrete mixture will be agitated by the fin segments 37, 39 and 41 when mixing drum 18 is caused to rotate in a first direction, while the fin segments will urge the mixture toward the discharge mechanism 24 when the rotational direction of the mixing drum 18 is reversed. The forward curving fin segment 39 acts to help lift and toss the mixture toward the middle of the drum 18 when the drum 18 is rotated to mix the material. The various sections of spiral fin assembly 36 are secured in the mixing drum 18 in a conventional manner.
As can be seen, particularly in FIG. 1, concrete mixing drums of conventional design have had a mixing fin assembly 36 which forms spiral mixing flights located symetrically opposite and rotated 180.degree. apart. The two flights are usually identical to each other in most conventional designs, especially in the discharge portion. As stated, the rotation of the drum in a given direction allows material to be loaded into the drum and agitated, while rotation of the drum in the opposite direction will discharge the concrete as it slides off the flighting toward the outlet end. The pitch of the flights varies from one end to the other as needed for optimum performance and the type of action needed in any given area. Particularly in the small end of the drum where the material is loaded into and discharged out of the drum, the spiral spacing and pitch is quite critical. If the pitch be too coarse, the slope of the flight where the concrete is sliding down the flight toward the discharge does not have enough slope angle to slide easily on the flight surface. On the other hand, if the pitch be too fine, the slope is more steeply inclined, but the flights are in much closer spacing with the adjacent, opposite fin. When concrete mixtures with high viscosity and very thick consistencies are used, the concrete will become wedged in between the flights and be difficult to discharge.
It is well known in the art that concrete mixes which have very thick consistencies normally provide higher strength cured concrete and so thick consistency material is generally preferred. It has been a long standing problem in the art of concrete mixing drum technology to arrive at the best spiral pitch when using concrete mixes which have very thick consistencies because, as can be seen from the above, changing the pitch angle either way can cause difficulties with respect to the ability of the mixing drum to discharge the contents. Thus, there remains a need to provide better concrete discharge characteristics particularly with regard to mixes having a thick is consistency in rotary mixing drums. A further goal of the design of portable rotary mixing drums or vessels is to reduce the weight of empty vessels so that additional concrete may be carried without exceeding vehicle load limits.
Thus, a primary object of the present invention to improve the overall performance characteristics of concrete mixing fins in inclined axis rotary drum mixers.
Another object of the present invention is to reduce the overall weight of inclined axis rotary drum mixers carried by cement mixing vehicles.
A further object of the present invention is to improve the ability of inclined axis rotary drum mixers to control the discharge of concrete mixtures over a range of thicknesses or slope values.
A still further object of the present invention is to improve the ability of inclined axis rotary drum mixers to discharge very thick concrete mixtures and particularly the material at the end of a batch.
A yet still further object of the present invention is to reduce material costs and installation expenses in the manufacture and maintenance of inclined axis rotary drum mixers.
Still another object of the present invention is to improve the ability of inclined axis rotary drum mixers to control the discharge of thin concrete mixtures.
Yet another object of the present invention is to reduce the maintenance associated with mixing fins in inclined axis rotary drum mixers, particularly those carried by cement mixing vehicles.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages and the objects obtained by its use, reference should be made to the drawings and to the accompanying descriptive matter which illustrates a detailed embodiment of the invention.